Field of the Invention
The present invention relates to an optical fiber and a light source device including an optical fiber.
Description of the Related Art
High-power signal light propagated in an optical fiber causes a nonlinear optical phenomenon such as four-wave mixing (FWM), self-phase modulation (SPM), or cross-phase modulation (XPM). In optical fiber communications, such a nonlinear optical phenomenon deteriorates the quality of signal light and limits the capacity of information propagatable. Therefore, optical fibers for communications are desired not to cause the nonlinear optical phenomenon.
Meanwhile, there has been a proposal of an applied technology in which the nonlinear optical phenomenon that occurs in an optical fiber is utilized positively. Specifically, a silica-glass-based highly nonlinear optical fiber (HNLF) has been in development as an optical fiber suitable for efficiently causing the nonlinear optical phenomenon. The HNLF is employed in various applied technologies such as fiber lasers; wideband low-noise optical amplification; supercontinuum (SC) light sources; optical signal processing; distortion and temperature sensors; measurement of frequency, time, length, and so forth; and near-infrared spectroscopy.
A nonlinear coefficient γ[1/W/km] representing the degree of nonlinearity of an optical fiber is defined as follows:γ=n2/Aeff×2π/λ,  (1)where n2 denotes the nonlinear refractive index [m2/W] of glass, Aeff denotes the effective area [μm2] of the optical fiber, and λ denotes the wavelength [nm].
One of the applied technologies employing the HNLF is an optical frequency comb, which is generated as follows. Seed light having a single or a few wavelengths is inputted to the HNLF, whereby FWM occurs in the HNLF. Consequently, multiwavelength light having wavelengths at regular intervals is outputted from the HNLF. The optical frequency comb has long been studied for some uses such as measurement and spectroscopy. In recent years, application of the optical frequency comb to a multiwavelength light source for large-capacity wavelength-division-multiplexing (WDM) transmission has been being examined.
There is a proposal of a dispersion-flattened HNLF, which causes FWM with high efficiency and generates a high-quality optical frequency comb over a wide wavelength range around 1550 nm, which is a waveband used for optical communications. In the dispersion-flattened HNLF, the absolute value of chromatic dispersion is small and the dispersion slope is substantially zero (the chromatic dispersion becomes maximal) around the wavelength of 1550 nm. That is, the absolute value of chromatic dispersion is suppressed to be small over a wide waveband. In this specification, a wavelength at which the dispersion slope becomes zero and the chromatic dispersion becomes maximal is referred to as “peak wavelength” [nm], and the chromatic dispersion at the peak wavelength is referred to as “peak dispersion” [ps/nm/km].
Masaaki Hirano et al. discuss, in “Silica-based Highly Nonlinear Fiber Advances,” OFC2016, Tu2E.4 (2016), an HNLF having a peak wavelength in the waveband for optical communications with a peak dispersion of zero (see HNLF-F in Table 1, and Type-III in FIG. 2).
In JP2005-331818A, a dispersion-flattened HNLF having a peak wavelength of 1550 nm with a peak dispersion of zero is disclosed in FIGS. 2 and 7 and Tables 1 and 2, and another HNLF in which the chromatic dispersion at 1550 nm is substantially zero but the peak wavelength is longer than or shorter than 1550 nm is disclosed in FIG. 11 and Table 3. These HNLFs each exhibit an anomalous peak dispersion (a positive chromatic dispersion).